Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of measurements made during the drilling of wellbores. More specifically, the invention relates to methods for determining rheological properties of well fluid or drilling mud as they exist down in a wellbore during the actual drilling of the wellbore.
2. Background Art
Drilling wellbores for purposes such as petroleum production is generally performed using rotary drilling techniques. A rotary drill bit is coupled to one end of a drilling tool assembly, and the assembly is rotated while applying axial force to the bit. Drilling fluid, called xe2x80x9cdrilling mudxe2x80x9d is pumped through the interior of the drilling tool assembly, and outward through appropriately selected orifices, called nozzles or jets, disposed in the body of the drill bit. The drilling mud that is discharged through the jets performs a number of functions, including cooling and lubricating the drill bit, applying hydrostatic pressure to the earth formations that have been drilled in order to control pressure of fluids in the pore spaces of the formations, and lifting drill cuttings generated by the bit to the earth""s surface through an annular space between the drilled wellbore and the exterior of the drilling tool assembly.
To perform all these functions adequately, the hydraulic properties of the drilling mud, known in the art as xe2x80x9crheologyxe2x80x9d or xe2x80x9cTheological propertiesxe2x80x9d, must be carefully controlled. Generally speaking, the rheological properties of the drilling mud are selected to meet a variety of performance criteria. These performance criteria include, for example that a circulating pressure loss in the annular space is not excessive, so that the drilling mud has an adequate capacity to lift the drill cuttings out of the wellbore, that excessive xe2x80x9cswabxe2x80x9d and xe2x80x9csurgexe2x80x9d pressures (reduced and excess hydrostatic pressures, respectively, deep in the wellbore, caused by movement of the drilling tool assembly out of and into the wellbore acting as a syringe) are avoided, and that erosion of susceptible clay-bearing earth formations is minimized, among other factors.
Drilling mud compositions commonly used in wellbore drilling have pseudo-plastic behavior under applied stresses. Various mathematical models have been developed to describe this behavior. One such model is known as the xe2x80x9cPower lawxe2x80x9d model. Another model is known as the xe2x80x9cBingham Plasticxe2x80x9d model. A fluid modeled using the Bingham Plastic model is presumed not to flow until an applied shear stress, xcfx84, exceeds a minimum value xcfx840. This minimum value of shear stress is known as the xe2x80x9cyield pointxe2x80x9d (YP) of the particular drilling mud. At stress levels above the YP, changes in shear stress become proportional to the changes in the shear rate. The proportionality constant is known as the plastic viscosity (PV), represented in the equation below as xcex3. The Bingham Plastic model can be represented by the following expression:
xcfx84=xcfx840+xcexc0xcex3;xcfx84xe2x89xa7xcfx840 
There are various instruments known in the art for determining PV and YP for a particular drilling mud. Typically, these instruments are used at the earth""s surface wherein a sample of the drilling mud is placed in the instrument and the properties of the drilling mud are determined. One such instrument is called a rotational, or Fann, viscosimeter. Typically, the Theological properties of mud are determined at surface temperature and pressure. The rheological properties which exist at the bottom of a wellbore as the wellbore is being drilled may be substantially different than those determined at the earth""s surface. Various factors, including temperature, flow rate, hydrostatic pressure, presence of drill cuttings and dilution by fluids in the formations moving into the wellbore may cause the Theological properties downhole to be different than those measured at the earth""s surface. More advanced rheometers are available for measuring the Theological parameters at elevated temperatures and pressures, however, they can only approximate the conditions at depth in a wellbore (xe2x80x9cdownholexe2x80x9d) and do not yet extend to the high pressure regimes encountered in many wells.
On method known in the art for estimating rheological properties downhole is described, for example, in, In-Situ Characterization of Drilling Mud, Maglione et al., SPE Journal, vol. 5, no. 4 (December 2000). The method described in this reference includes measuring standpipe pressure and calculating an estimate of average values of mud parameters over the entire mud circulation system. The method disclosed in this reference, however, does not disclose how to make any explicit determination of mud Theological properties at or near the bottom of the wellbore, and certainly does not disclose how to do so during the actual drilling of the wellbore.
What is needed is a method for determining downhole drilling mud rheological properties that can be carried out substantially in real time as a wellbore is being drilled.
One aspect of the invention is a method is for determining Theological properties of a wellbore fluid. The method includes moving the wellbore fluid through a wellbore tool assembly into a wellbore at a first flow rate. A pressure is measured at a first position proximate a bottom of the wellbore in an annular space between the interior wall of the wellbore and the exterior of the wellbore tool assembly. A pressure is measured at a second position in the annular space axially separated from the first position. The wellbore fluid is then moved through the wellbore tool assembly at a second flow rate. Measuring pressure at the first and second positions is repeated, and at least one Theological property is determined from the pressure measurements.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.